Changes in the needs of laboratories who use electron microscopes as investigation tools, and in particular changes in themes towards studying materials at nanometric scale are requiring increasingly high-performance investigation means. In electron microscopy, the techniques used are changing towards quantitative use of images, requiring acquisition methods to be developed.
An electron energy loss spectrum is made up mainly of two zones, namely a low-loss (LL) zone, which is generally a high-intensity zone and includes the zero-loss reference spectrum line followed by plasmons, and a core-loss (CL) zone which usually extends from a few hundred to a few thousand electron volts (eV). The ratio of the intensities between the two zones can reach 109 and does not make it possible, in the prior art, to record the entire spectrum simultaneously.
The detectors currently used for recording such spectra are of the Charge Coupled Device (CCD) type and are highly cooled, thick (front-illuminated), or thin (back-illuminated) when better efficiency is sought or when the wavelength so requires (wavelength shorter than 450 nm).
The best detectors currently available offer a dynamic range of the order 10,000, and highly exceptionally it is possible to approach 100,000, i.e. respectively a factor of 100,000 or of 10,000 short of being capable of sensing simultaneously intensity ratios of 109. The five or four decade shortfall requires the spectrum to be recorded in two independent sequences. For recording the low-loss zones, the recording starts with exposure for a very short time (100 μs, for example). If the number of electrons of the spectrum is high, then, in order to avoid problems of saturation, the detector is generally read line-by-line, which takes a non-negligible length of time, and then the detector is rinsed completely, and a second exposure is effected in order to record the core-loss zone, with a much longer exposure time in order to obtain a significant signal. In which case, it is observed that the low-loss zone of the spectrum can be very highly saturated. At the end of the second exposure, the detector is read by summing the contents of the lines portion by portion (summing referred to as “partial binning”) or of all of the lines (summing referred to as “total binning”) as a function of the pixel fill rate.
All of these operations require the image-taking to be slow, which adversely affects acquisition of an electron energy loss spectrum and, in particular, prevents fast acquisition of electron energy loss spectra of very large dynamic range.
In addition, prior art devices require the power source to remain stable (100,000 to 1,000,000 volts) for one minute, or even longer, making them vulnerable to the slightest variations in power supply voltage.
Finally, prior art devices do not make it possible to obtain high precision in positioning the energy peak, which precision is necessary for identifying the characteristic distributions in the energy loss spectrum.